JP5145342B2 - Method for forming transparent conductive film - Google Patents

Description

The present invention relates to a method for forming a transparent conductive film.
This application is based on Japanese Patent Application No. 2007-218296, the contents of which are incorporated herein.

ITO (In ₂ O ₃ —SnO ₂ ), which is a transparent conductive material, is used as an electrode for solar cells and light emitting diodes. However, indium (In), which is a raw material for ITO, is a rare metal and is expected to increase in cost due to difficulty in obtaining it. Therefore, an abundant and inexpensive ZnO-based material has attracted attention as a transparent conductive material replacing ITO (for example, see Patent Document 1 below). ZnO-based material is suitable for uniform deposition of possible sputtering a large substrate, it is possible to easily deposited by changing the target of In ₂ O ₃ based material. Further, the ZnO-based material does not contain a lower oxide (InO) having a high insulating property unlike the In ₂ O ₃ -based material.
JP-A-9-87833

Although the ZnO-based material is a material having a resistance lower than that of ITO, the general specific resistance is 500 μΩcm to 1000 μΩcm, which is 2.5 to 5 times that of ITO. Therefore, further reduction in resistance of the ZnO-based material is desired.
Further, the ZnO-based material has a property of increasing its specific resistance when oxidized in the atmosphere at a high temperature. Thus, since the ZnO-based material has low heat resistance, there is a problem that cooling is necessary before the ZnO film formed by heating in vacuum is taken out into the atmosphere.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a method for forming a transparent conductive film made of a ZnO-based material having a low specific resistance and excellent heat resistance.

  The inventor of the present application has found that the sputtering voltage and the magnetic field strength during film formation affect the specific resistance of the ZnO-based film. It has been conventionally known that the specific resistance of a ZnO-based film varies greatly depending on the film thickness and the degree of oxidation. However, since the noise due to variations in the film thickness and the degree of oxidation is large, the dependence of the specific resistance on the sputtering voltage and the magnetic field strength are known. The dependency was not confirmed. The inventor of the present application discovered the dependence of the specific resistance on the sputtering voltage and the magnetic field strength for the first time in developing a method for forming a thick ZnO-based film as a transparent electrode for solar cells.

The present invention provides a transparent conductive film having ZnO as a basic constituent element on a substrate by performing sputtering by applying a horizontal magnetic field to the surface of the target while applying a sputtering voltage to a target having a transparent conductive film forming material. Using a magnetic field generating means disposed on the back surface of the target, the maximum horizontal magnetic field strength of the target surface is 1500 gauss or more, the sputtering voltage is 300 V or less , and a magnetic field generating means by a zigzag motion in the target and the plane parallel to, have rows the sputtering in the deposition chamber which is a high vacuum evacuation, the transparent conductive film formed by the sputtering, in the atmosphere, 500 Annealing is performed at a temperature of ℃ or higher.
According to the method for forming a transparent conductive film of the upper SL, it is possible to form a ZnO-based film with well-defined crystalline lattice can be resistivity obtain a low transparent conductive film. In addition, since a ZnO-based film with a crystal lattice is formed, it is difficult to oxidize even when heated to a high temperature, and a transparent conductive film with excellent heat resistance can be obtained.
Moreover, according to the method for forming the transparent conductive film, the erosion region of the target can be dispersed, and the durability of the target can be improved.

Note that as the material for forming the transparent conductive film of the target, a material in which a substance containing Al is added to ZnO may be used.
In this case, a transparent conductive film having a particularly low specific resistance can be obtained among the ZnO-based films.

Further, the sputtering may be performed while introducing oxygen gas.
In this case, since an oxygen-rich ZnO-based film is formed, a transparent conductive film with high light transmittance can be obtained.

Further, the magnetic field generating means for generating the horizontal magnetic field includes a first magnet having a first polarity and a second magnet having a second polarity arranged along the back surface of the target; the second magnet has the first magnet Arranged to surround the magnet; a configuration may be employed.
In this case, since a strong horizontal magnetic field can be generated on the surface of the target, it becomes possible to form a ZnO-based film with an aligned crystal lattice. Therefore, a transparent conductive film having a low specific resistance and excellent heat resistance can be obtained.

Further, the sputtering may be performed while changing a relative position between the substrate and the target.
In this case, a uniform transparent conductive film can be obtained for the entire substrate.

The sputtering voltage may be applied using both a DC power source and an RF power source.
In this case, the sputtering voltage can be reduced. As a result, it becomes possible to form a ZnO-based film with an organized crystal lattice, and a transparent conductive film with a low specific resistance can be obtained.

  According to the present invention, it becomes possible to form a ZnO-based film with an organized crystal lattice, and a transparent conductive film having a low specific resistance and excellent heat resistance can be obtained.

FIG. 1 is a schematic configuration diagram of a magnetron sputtering apparatus according to an embodiment of the present invention. FIG. 2 is a plan sectional view of the film forming chamber. FIG. 3 is a front view of the sputtering cathode mechanism. FIG. 4 shows a modification of the magnetron sputtering apparatus. FIG. 5 is a graph showing the relationship between the horizontal magnetic field strength and the sputtering voltage. FIG. 6 is a graph showing the relationship between the thickness of the ZnO-based film and the specific resistance. FIG. 7A is a graph showing the relationship between annealing temperature and specific resistance. FIG. 7B is a graph showing the relationship between the annealing temperature and the specific resistance. FIG. 8 is a graph showing the relationship between the sputtering voltage and the specific resistance.

Explanation of symbols

5 Substrate 10 Magnetron sputtering device 22 Target 26 DC power supply (voltage application means)
30 Magnetic circuit (magnetic field generating means)
31 1st magnet 32 2nd magnet

A method for forming a transparent conductive film according to an embodiment of the present invention will be described below with reference to the drawings.
(Magnetron sputtering equipment)
FIG. 1 is a schematic configuration diagram of a magnetron sputtering apparatus. Sputtering apparatus 10 of the present embodiment is a interback type sputtering apparatus, and a charging / unloading chamber 12 of the substrate (not shown), and a deposition chamber 14 with respect to the substrate. The preparation / extraction chamber 12, is connected to roughing exhaust unit 12p such as a rotary pump, the film deposition chamber 14, a high vacuum evacuation unit 14p, such as a turbo-molecular pump is connected. In the sputtering apparatus 10 of the present embodiment, the substrate vertical support to be carried into the charging / unloading chamber 12, evacuating the feed / take-out chamber 12 in roughing vacuum means 12p. Next, the substrate is transferred into the film forming chamber 14 evacuated by the high vacuum evacuation means 14p, and a film forming process is performed. The substrate after film formation is carried out to the outside via the preparation / removal chamber 12.

A gas supply unit 17 for supplying a sputtering gas such as Ar is connected to the film forming chamber 14. It is also possible to supply a reactive gas such as O ₂ from the gas supply means 17. A sputtering cathode mechanism 20 is disposed vertically in the film forming chamber 14.
FIG. 2 is a plan sectional view of the film forming chamber. The sputter cathode mechanism 20 is disposed on one side surface in the width direction of the film forming chamber 14. A heater 18 for heating the substrate 5 is disposed on the other side surface of the film forming chamber 14.

The sputter cathode mechanism 20 mainly includes a target 22, a back plate 24, and a magnetic circuit 30. The back plate 24 is connected to a DC power source 26 and is held at a negative potential. On the surface of the back plate 24, a target 22 is disposed by bonding a material for forming a ZnO-based film with a brazing material. The material for forming the ZnO-based film may be only ZnO, or a material obtained by adding a predetermined material to ZnO.
A sputtering gas is supplied from the gas supply means 17 to the film forming chamber 14, and a sputtering voltage is applied to the back plate 24 by the DC power source 26. The ions of the sputtering gas excited by the plasma in the film forming chamber 14 collide with the target 22 and eject atoms of the forming material of the ZnO-based film. By attaching the jumped out atoms to the substrate 5, a ZnO-based film is formed on the substrate 5.

A magnetic circuit 30 that generates a horizontal magnetic field on the surface of the target 22 is disposed along the back surface of the back plate 24. The magnetic circuit 30 includes a first magnet 31 and a second magnet 32 having different polarities on the surface on the back plate 24 side. The first magnet 31 and the second magnet 32 are both permanent magnets.
FIG. 3 is a rear view of the sputtering cathode mechanism. The first magnet 31 has a linear shape, and the second magnet 32 has a frame shape surrounding the first magnet 31 at a predetermined distance from the peripheral edge. The first magnet 31 and the second magnet 32 are attached to the yoke 34 to form a magnetic circuit unit 30a. A plurality (two in this embodiment) of magnetic circuit units 30 a and 30 b are connected by a bracket 35 to constitute the magnetic circuit 30.

  As shown in FIG. 2, a magnetic field represented by a line of magnetic force 36 is generated by the first magnet 31 and the second magnet 32 having different polarities on the back plate 24 side. As a result, a position 37 where the vertical magnetic field is 0 (the horizontal magnetic field is maximum) is generated on the surface of the target 22 between the first magnet 31 and the second magnet 32. By generating high-density plasma at this position 37, the film forming speed can be improved.

  At this position 37, the target 22 erodes most deeply. In order to improve the use efficiency (life) of the target by preventing the position 37 from being fixed, and to improve the arcing and the like by increasing the cooling efficiency of the target and the cathode, the magnetic circuit 30 is formed to be swingable in the horizontal direction. Has been. In addition, since the erosion is rectangular or semicircular at the upper and lower ends of the target 22, the magnetic circuit 30 can swing in the vertical direction. Specifically, a pair of actuators (not shown) that reciprocate the bracket 35 of the magnetic circuit 30 independently in the horizontal direction and the vertical direction are provided. By driving these horizontal and vertical actuators at different periods, the magnetic circuit 30 performs a zigzag motion in a plane parallel to the target 22.

(Modification)
FIG. 4 shows a modification of the magnetron sputtering apparatus. The sputtering apparatus 100 is an in-line type sputtering apparatus, and includes a preparation chamber 12, a film formation chamber 14, and a take-out chamber 16 in this order. In this sputtering apparatus 100, the substrate 5 is supported vertically and carried into the preparation chamber 12, and the inside of the preparation chamber 12 is exhausted by the roughing exhaust means 12p. Next, the substrate is transferred into the film forming chamber 14 evacuated by the high vacuum evacuation means 14p, and a film forming process is performed. The substrate 5 after film formation is carried out from the take-out chamber 16 evacuated by the roughing evacuation means 16p.

  In the film forming chamber 14, a plurality (three in this modification) of sputtering cathode mechanisms 20 are arranged side by side in the transport direction of the substrate 5. Each sputter cathode mechanism 20 is configured in the same manner as in the above embodiment. In the present modification, a ZnO-based film is formed on the surface of the substrate 5 by each sputtering cathode mechanism 20 in the process in which the substrate 5 passes in front of the plurality of sputtering cathode mechanisms 20. This makes it possible to form a homogeneous ZnO-based film and improve the throughput of the film forming process.

(First embodiment)
In this embodiment, a ZnO (AZO) film to which Al is added is formed using the sputtering apparatus shown in FIGS. The ZnO-based film exhibits conductivity by forming oxygen vacancies in the crystal and releasing free electrons. Since this ZnO-based film is very easily oxidized, it is desirable to perform heat deposition in order to reduce the influence of the oxidation source by degassing. In addition, the ZnO-based film has a property that conductivity is improved by allowing B, Al, Ga, or the like to enter the position of Zn in the crystal and emitting free electrons as ions. Also from this point of view, it is advantageous to perform heating film formation in which migration easily occurs.

The target 22 shown in FIG. 2 employs ZnO to which Al ₂ O ₃ is added in an amount of 0.5 wt% to 10.0 wt% (2.0 wt% in this embodiment) as a material for forming the transparent conductive film. The alkali-free glass substrate 5 is carried into the film forming chamber 14, and the substrate 5 is heated to 100 ° C. to 600 ° C. (200 ° C. in this embodiment) by the heater 18. The film formation chamber 14 is evacuated by high vacuum exhaust means, Ar gas is introduced as a sputtering gas from the gas supply means, and the pressure in the film formation chamber 14 is maintained at 2 mTorr to 10 mTorr (5 mTorr in this embodiment). While swinging the magnetic circuit 30 (in this embodiment 4W / cm ²⁾ Power density ^1W / cm ^{2 ~8W} / cm 2 on the back plate 24 by the DC power supply 26 turning on the power. Note that annealing is not performed after film formation because heat film formation is performed, but annealing may be performed after film formation.

As mentioned above, ZnO-based films, such as B or Al, Ga enters the position of Zn in the crystal, that emits free electrons as ions, have the property of electrical conductivity is improved. Therefore, sputtering is performed using a ZnO target to which Al ₂ O ₃ is added, and a ZnO (AZO) film to which Al is added is formed, thereby obtaining a transparent conductive film having a particularly low specific resistance among ZnO-based films. be able to.

  The inventors of the present application evaluated the magnetic field strength dependence of the specific resistance of the ZnO-based film. Therefore, the first level in which the magnetic circuit 30 is adjusted so that the horizontal magnetic field strength on the target surface is 300 gauss and the second level in which the magnetic circuit 30 is adjusted so that the horizontal magnetic field strength on the target surface is 1500 gauss. A ZnO-based film was formed. The specific resistance was measured by setting the thickness of the ZnO-based film to 2000, 5000, 10000, and 15000 for each level.

  FIG. 5 is a graph showing the relationship between the horizontal magnetic field strength and the sputtering voltage. As shown in the figure, the higher the horizontal magnetic field strength, the lower the sputtering voltage. In general, the sputtering voltage is affected by the discharge impedance (= target voltage / target current), and the discharge impedance is affected by the magnetic field intensity on the target surface. Increasing the magnetic field strength increases the plasma density, resulting in a decrease in sputtering voltage. The first level (horizontal magnetic field strength is 300 gauss) sputtering voltage is about 450V, and the second level (horizontal magnetic field strength is 1500 gauss) sputtering voltage is about 300V.

FIG. 6 is a graph showing the relationship between the thickness of the ZnO-based film and the specific resistance. Since the specific resistance of the ZnO-based material has film thickness dependence, the specific resistance decreases as the film thickness increases.
The specific resistance of the ZnO-based film formed at the second level (1500 gauss, 300V) is smaller than that of the first level (300 gauss, 435V). The reason is considered as follows. Since the specific resistance is dependent on the film thickness, the ZnO-based material has a property that the crystal lattice is not easily aligned. A ZnO-based film formed with a high sputtering voltage (weak magnetic field) has a high specific resistance because the crystal lattice is disturbed. Even in this case, there is a tendency that by increasing the film thickness, the crystal lattice is arranged and the specific resistance decreases. However, since're way of the crystal lattice is not sufficient, as compared with a thin ZnO-based film with formed film thickness at a low sputtering voltage (strong magnetic field), the specific resistance is high.

FIG. 8 is a graph showing the relationship between the sputtering voltage and the specific resistance when the substrate is heated to 200 ° C. and a ZnO-based film having a film thickness of 2000 mm is formed (the sputtering voltage is described as a negative potential). ) Although the absolute value of the specific resistance in the following range 340V sputtering voltage is around 400Myuomegacm, the absolute value of the sputtering voltage is understood that the specific resistance exceeds 340V rapidly increases.
Accordingly, the sputtering voltage is less 340 V, performs sputtering as the maximum value of the horizontal magnetic field strength on the target surface 600 gauss or more (see FIG. 5), it is desirable to form the ZnO-based film. As a result, it is possible to form a ZnO-based film with an organized crystal lattice, and a ZnO-based film having a low specific resistance (a specific resistance of 500 μΩcm or less even when the film thickness is small) can be obtained. Further, it is possible to suppress by performing sputtering at a low voltage below 340 V, the damage is negative ions accelerated excited entered the substrate base film or the like by plasma is generated.
The lower limit of the sputtering voltage is a discharge voltage at which sputtering is possible. Further, the maximum value of the horizontal magnetic field strength is preferably 600 gauss or more as described above. As the maximum value of the horizontal magnetic field strength is larger, the discharge voltage can be lowered. However, since a permanent magnet is usually used for forming a magnetic field, the upper limit value is determined by the performance of the permanent magnet used.

  Further, the inventors of the present application evaluated the magnetic field strength dependence of the heat resistance of the ZnO-based film. Specifically, a ZnO-based film having a thickness of 5000 mm was formed at the first level and the second level, and after the film formation, annealing treatment was performed at various temperatures to measure the specific resistance. The annealing treatment was performed in the atmosphere for 1 hour at a temperature of 150 ° C. to 600 ° C. (every 50 ° C.).

  7A and 7B are graphs showing the relationship between the annealing temperature and the specific resistance, FIG. 7A is a graph of 350 ° C. or lower, and FIG. 7B is a graph of 350 ° C. or higher. When the annealing temperature is 450 ° C. or lower, there is no significant increase in specific resistance at both the first level and the second level. On the other hand, as shown in FIG. 7B, when the annealing temperature is 500 ° C. or higher, the specific resistance of the ZnO-based film of the second level (1500 gauss, 300 V) is higher than that of the first level (300 gauss, 435 V). Is also getting smaller.

  The reason is considered as follows. The ZnO-based film exhibits conductivity by forming oxygen vacancies in the crystal and releasing free electrons. As described above, the crystal lattice of the ZnO-based film formed with a high sputtering voltage (weak magnetic field) is disturbed, but the more disordered the crystal lattice, the easier it is to bond with oxygen. Therefore, a ZnO-based film formed with a high sputtering voltage (weak magnetic field) is easily oxidized by high-temperature annealing after film formation, and has a higher specific resistance than a ZnO-based film formed with a low sputtering voltage (strong magnetic field).

Therefore, it is desirable to form the ZnO-based film by performing sputtering with a sputtering voltage of 340 V or less (or less than 340 V) and a maximum horizontal magnetic field strength on the target surface of 600 gauss or more as described above. Thus, the ZnO-based film with well-defined crystalline lattice can be formed, even if annealing is performed at a high temperature after the deposition less likely to be oxidized, it is possible to suppress an increase in resistivity. That is, a ZnO-based film having excellent heat resistance can be obtained.
Accordingly, even when the substrate after heat deposition is taken out into the atmosphere, the cooling of the substrate can be abolished or simplified, and the manufacturing cost can be reduced.

(Second Embodiment)
In the second embodiment, an oxygen-rich ZnO film is formed.
The target 22 shown in FIG. 2 employs ZnO as a material for forming the transparent conductive film. The alkali-free glass substrate 5 is carried into the film formation chamber 14, and the substrate 5 is heated to 100 ° C. to 600 ° C. by the heater 18. The film formation chamber 14 is evacuated by high vacuum evacuation means, Ar gas is supplied at 50 sccm to 400 sccm as sputtering gas from the gas supply means, and O ₂ gas is supplied at 0 sccm to 20 sccm as reaction gas. The pressure in the film forming chamber 14 is maintained at 2 mTorr to 10 mTorr. While swinging the magnetic circuit 30, turning on the power of the power density of ^1W / cm ^{2 ~8W} / cm 2 on the back plate 24 by the DC power source 26.

In this way, an oxygen-rich ZnO film can be formed by performing sputtering while supplying O ₂ gas. Although the oxygen-rich ZnO film has a large specific resistance, it has a high light transmittance. Thereby, the transparent conductive film excellent in the optical characteristic can be obtained.

The technical scope of the present invention is not limited to the above-described embodiments, and includes various modifications made to the above-described embodiments without departing from the spirit of the present invention.
That is, the specific materials and configurations described in the embodiments are merely examples, and can be changed as appropriate.
For example, in the sputtering apparatus of the above-described embodiment, sputtering is performed while supporting the substrate vertically, but it is also possible to implement the present invention with a sputtering apparatus that horizontally supports the substrate.
Further, the magnetic circuit unit of the embodiment has been a second magnet of the second polarity and disposed around the first magnet of the first polarity, in addition to this, the first polarity around the second magnet You may comprise a magnetic circuit unit by arrange | positioning a 3rd magnet.

Moreover, although the DC power source is employed in the sputtering cathode mechanism of the above-described embodiment, it is possible to use both a DC power source and an RF power source. In the case of employing a DC power supply alone, as shown in FIG. 8, the specific resistance of the ZnO-based film formed by sputtering voltage 300 V (film thickness 2000 Å) was 436.6Myuomegacm. On the other hand, for example, when a DC power source with a low current of 4 A and a 350 W RF power source are used in combination, a ZnO-based film (film) is formed with a sputtering voltage for a ZnO-2 wt% Al ₂ O ₃ target of about 100 V. The specific resistance at a thickness of 2000 mm was 389.4 μΩcm. As described above, when the RF power source is used in combination with the DC power source, the sputtering voltage is lowered, and the specific resistance of the ZnO-based film is also lowered as the sputtering voltage is lowered. That is, the resistance of the ZnO-based film can be reduced not only by the magnetic field intensity but also by reducing the sputtering voltage from the power supply surface.

  According to the present invention, it is possible to provide a method for forming a transparent conductive film made of a ZnO-based material having a low specific resistance and excellent heat resistance.

Claims (6)

A method for forming a transparent conductive film containing ZnO as a basic constituent element on a substrate by applying a sputtering voltage to a target having a transparent conductive film forming material while generating a horizontal magnetic field on the surface of the target and performing sputtering. Because
Using the magnetic field generating means disposed on the back surface of the target, the maximum horizontal magnetic field strength of the target surface is set to 1500 gauss or more, the sputtering voltage is set to 300 V or less , and the magnetic field generating means is parallel to the target. and zigzag exercised in such a plane, have a row the sputtering in the deposition chamber which is a high vacuum evacuation, the transparent conductive film formed by the sputtering, in the atmosphere, an annealing treatment with 500 ° C. or higher temperature A method for forming a transparent conductive film, which is performed . A method for forming a transparent conductive film according to claim 1,
As a material for forming the transparent conductive film of the target, a material in which a substance containing Al is added to ZnO is used. A method for forming a transparent conductive film according to claim 1,
The sputtering is performed while introducing oxygen gas. A method for forming a transparent conductive film according to claim 1,
The magnetic field generating means for generating the horizontal magnetic field includes a first magnet having a first polarity and a second magnet having a second polarity arranged along a back surface of the target;
The second magnet is disposed so as to surround the first magnet. A method for forming a transparent conductive film according to claim 1,
The sputtering is performed while changing the relative position between the substrate and the target. A method for forming a transparent conductive film according to claim 1,
The sputtering voltage is applied using both a DC power source and an RF power source.

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